The present invention relates to a hydraulic control device of a forklift truck and more specifically to a hydraulic control device to control a raising/lowering hydraulic cylinder and an operating hydraulic cylinder.
In a conventional forklift truck, a hydraulic cylinder is employed as the mechanism for operating the forks and the masts. For example, Japanese Patent Application Publication No. 2014-97853 discloses a hydraulic control device having a hydraulic pump and an electric motor driving the hydraulic pump. By driving the hydraulic pump, the lift cylinder (raising/lowering hydraulic cylinder) for operation to lift the forks and the tilt cylinder (operating hydraulic cylinder) for operation to tilt the masts are operated.
In such a forklift truck, regenerative operation may be performed by returning hydraulic oil from a lift cylinder to a hydraulic pump thanks to the weight of a load during operation to lower the forks. The above-described Publication discloses the hydraulic control device having a control valve disposed in an oil pipe connecting a bottom chamber of the lift cylinder to the hydraulic pump and a pilot type flow control valve disposed in a bypass pipe to return hydraulic oil flowed through the control valve to a tank. The hydraulic oil is discharged from the lift cylinder during operation to lower the forks is distributed to the hydraulic pump and the tank by the flow control valve.
For example, in the case of simultaneously performing operation to lower the forks and operation to tilt the masts, the opening of the flow control valve is controlled based on the pressure introduced through a pilot passage to control the flow of the hydraulic oil so that each operation is performed at the indicated speed. As a result, the tilt cylinder is driven by the hydraulic oil discharged from the hydraulic pump to tilt the masts while the hydraulic oil is discharged from the bottom chamber of the lift cylinder and the forks are lowered by the suction of the hydraulic pump.
In the case of simultaneously performing the operation to tilt the masts during the operation to lower the forks, the rotational speed of the hydraulic pump (electric motor) may be varied rapidly because sole operation is shifted to simultaneous operation. Then, the flow control valve needs to control flow of the hydraulic oil required to operate at the indicated speed corresponding to each operation. The flow control valve needs to have a quick response to the shift of the operation.
The present invention which has been made in light of the above problems is directed to providing a hydraulic control device of a forklift truck which permits to improve a response of the flow control valve.